Piping made from a polymer of a lower olefin containing from 2 to about 4 carbon atoms are used in water distribution systems in which the temperature of the water is relatively low, typically less than 100° C., and the pressure of the water is less than about 790 kPa (100 psig). Such PO piping, and PEX in particular, is conventionally protected with antioxidants but is nevertheless susceptible to diffusion of oxygen and reaction with oxidizing agents entering the PO wall, both from fluid carried within the pipe and from the environment outside the pipe. To date, the extent of damage from within has been recognized and addressed by the addition of antioxidants and other formulation ingredients. To minimize diffusion of oxygen from the air into the pipe, a core of PEX is externally coated with a barrier layer of a material such as an ethylene vinyl alcohol copolymer (EVOH) using an intermediate adhesive layer, but the EVOH layer on such pipe fails to protect against oxidation from within the pipe; moreover, an EVOH layer is known to be susceptible to cracking when expanded. Adhesive is required because EVOH cannot be extruded over PE or polypropylene (“PP”) or polybutylene (“PB”) pipe under conditions such that the contiguous EVOH and PO surfaces are adequately self-adhered.
To overcome the mechanical limitations of PEX pipe adhesively bonded to a EVOH outer layer, a multilayer pipe is disclosed in WO 99/49254 to Johansson et al. Though no details are provided sufficient to enable one to determine the effectiveness of the combination without an undue amount of experimentation, a PEX core having an EVOH outer layer adhesively bonded (with an intermediate adhesive layer) is also coated with another layer of adhesive which is stated to prevent cracking of the pipe, which would otherwise occur, when the pipe was expanded. If one were to recognize the importance of protection from within, it is expected that one could adhesively bond a tubular (or annular) core of EVOH to the inner surface of PEX pipe, forming a barrier layer protecting the PEX against degradation from oxidizing agents and oxygen in oxygenated water containing deleterious oxidizing agents, except that EVOH hydrolyzes in water.
To cope with the problem of oxidation from within, one could extrude a twin-laminate pipe having a laminated wall formed by an outer layer of PO which is extruded over the exterior surface of a thin-walled inner tubular layer (or “core”) of a material which has desirable barrier properties the PO, even if cross-linked, does not have. For potable water systems, the core would be chosen to provide an effective barrier against all oxidizing agents typically present in potable water, which agents deleteriously react with the PO outer layer, particularly if it is cross-linked polyethylene (“PEX”). The thin-walled core would desirably have a lower permeability and lower diffusion coefficient for oxygen than PEX so that the outer layer is protected against degradation from oxygenated water within the pipe. In addition, the outer layer of the twin-laminate may again be protected against oxidation with a protective cover of barrier material. Barrier materials are typically adhesively bonded to the PO layer because directly bonding a conventional barrier layer such as EVOH to the PEX by co-extrusion produces unsatisfactory bonding.
An alternative piping system which is essentially immune to degradation by oxidizing agents and substantially impermeable to oxygen is available. Such pipe is made from either poly(vinyl chloride) (“PVC”) or chlorinated poly(vinyl chloride) (“CPVC”), the choice depending upon the temperature of the water and other “use” criteria. But it is well known that advantages of a PEX piping system are not available in a PVC and/or CPVC (“PVC/CPVC”) piping system, and vice versa. Accordingly much effort has been devoted to producing pipe which has the advantages of both systems and the drawbacks of neither. However, neither PVC nor CPVC is directly bondable to a polyolefin surface satisfactorily; and attempts to provide an intermediate adhesive layer have, to date, failed.
U.S. Pat. No. 6,124,406 discloses that a “blocky” chlorinated polyolefin (“b-CPO”) may be used to compatibilize PVC or CPVC with a polyolefin rubber (“PO-rubber”) and that a blend of PVC and/or CPVC with blocky chlorinated polyethylene (“b-CPE”) and a PO-rubber (familiarly referred to as an “elastomer” herein) can have a combination of good impact resistance, high heat distortion temperature (relative to the base CPVC or PVC), good tensile properties, oxidation resistance, and stability to ultraviolet light (UV) exposure. The term “polyolefin rubber” as used herein, refers to an olefinic rubber of polymerized lower (C2–C4) monoolefins, e.g. ethylene-propylene rubber, and/or an olefinic rubber which in addition, contains a polymerized diene, e.g. ethylene/propylene/ethylidene norbornene rubber referred to as EPDM rubber. The term “b-CPO” refers to blocky chlorinated PO having both, high Cl content PO blocks (e.g. 50% to 75% by weight Cl) and relatively non-chlorinated crystallizable PO blocks, the b-CPO having a residual crystallinity of at least 95% (calculated as indicated in the '406 patent), and being produced without appreciably swelling the PO or melting the crystalline phase, i.e. less than 10% increase in volume due to swelling of the precursor PO at 25° C. Details of the preparation of b-CPE are set forth in the '406 patent, the disclosure of which is incorporated by reference thereto as if fully set forth herein. Blocky chlorinated PP (“b-CPP”) and blocky chlorinated polybutene (“b-CPB”) are made in an analogous manner by chlorination at a temperature below the melting point of the resins. Reference to “polybutene” herein includes polyisobutene.
The '406 patent disclosed that randomly chlorinated polyethylene (“r-CPE”) functions as an adequate compatibilizer for PVC/CPVC and a PO-rubber, and that the properties of a blend made with b-CPE were better than those of a blend made with r-CPE. There is no comparable disclosure relating to partially randomly chlorinated polyethylene (“pr-CPE”). r-CPE may be prepared as disclosed in U.S. Pat. Nos. 3,110,709; 3,454,544; 3,563,974 or 5,525,679 to contain the desired amount of chlorine. r-CPE is rubbery and typically contains in the range from about 25% up to about 45% by weight bound Cl with heats of fusion in the range from about 0.1 to less than 15 cal/gm. r-CPE which is commercially available as Tyrin® is used as a coating for fabrics. pr-CPE may be prepared in a manner analogous to that disclosed for r-CPE above; pr-CPE may contain in the range from about 5% up to about 50% by weight bound Cl with heats of fusion in the range from about 15 to 50 cal/gm, and is distinguishable from b-CPE having a bound Cl content in the range from about 15 to about 50% in that the residual crystallinity of pr-CPE is less than that defined by the following equation% ▴HR=−0.068(% Cl)2+2.59(% Cl)+74.71in which ▴HR is the enthalphy of fusion of residual polyethylene crystallinity (see the '406 patent).
In the range from about 15% to 20% the residual crystallinity of r-CPE and pr-CPE is approximately equal. As long as the chlorine content of the chlorinated polyolefin is in the range from 5 to 50% the residual crystallinity is not critical though higher residual crystallinity material is preferred.
It will be appreciated that the values given for the foregoing chlorinated polyethylenes are particularly directed to cores coextrudable with PEX or PE. In an analogous manner, randomly chlorinated PP and PB; partially randomly chlorinated PP and PB; and blocky PP and PB may be prepared but will have correspondingly different chlorine contents.
Providing better impact properties of a blend of immiscible polymers, such as PVC/CPVC with a polyolefin rubber, was the thrust of the '406 patent, the use of a b-CPO being directed towards a specific function, namely, as a compatibilizer or interfacial agent which was unexpectedly superior to randomly chlorinated polyolefin (r-CPO), and by inference, to partially randomly chlorinated polyolefin (pr-CPO). The effectiveness of such an agent is determined by its properties, mainly its ability to control the size of the dispersed phase, stabilize the phase against coalescence, and increase interfacial adhesion between the immiscible phases, none of which properties is correlatable with the extrudability of b-CPO as an inner layer bondable to an outer layer of PO, or the effectiveness of b-CPO as a barrier to oxidant molecules. Irrespective of the particular mechanism by which an individual compatibilizing or interfacial agent may function, there is nothing to suggest that a b-CPO compatibilizing or interfacial agent would be bondingly co-extrudable under substantially the same processing conditions as a corresponding PO chosen to be co-extrudable under matching conditions. Neither is there anything to suggest that, when so co-extruded, either r-CPO, pr-CPO or b-CPO would form a cohesive bond with a corresponding PO; in particular, there is no suggestion that either r-CPE, pr-CPE or b-CPE forms a cohesive bond with PEX.
Such a “cohesive bond” is demonstrated by co-extruding a tri-layer pipe of PEX layers with a layer of b-CPE sandwiched between the PEX layers; pulling apart the PEX layers in a ring peel test (ASTM F1281-99 section 9.3.2) showed portions of b-CPE adhering to the PEX surfaces, indicating the bonds between molecules of PEX are stronger than the bonds between molecules of b-CPE. The same is true for sandwiched r-CPE and pr-CPE layers; and analogously, for randomly chlorinated, partially randomly chlorinated and blocky other lower polyolefins, whether polypropylene or polybutene. By “twin-layer” and “tri-layer” pipe, reference is made to the poly(lower)olefin layers and not to any layer of adhesive which might additionally be included.